1. Field of the Invention
The present invention relates to acrylic fiber suitable for producing carbon fiber or graphite fiber, and a process for producing the acrylic fiber.
2. Description of the Prior Art
Carbon fiber and graphite fiber (hereinafter collectively referred to as "carbon fiber) which are obtained using acrylic fiber as precursor are commercially produced and sold as reinforcing fiber materials for high-performance composite materials used in the aviation industry, the aerospace industry, sports goods, leisure goods, etc., because of their excellent mechanical properties. In the market, high-quality and inexpensive carbon fiber is required for improving the performance characteristics of these composite materials.
On the composition of a copolymer as a material for acrylic fiber used as a precursor of carbon fiber (hereinafter referred to as "precursor fiber" in some cases) and a spinning process of the copolymer, a large number of proposals have been made. For example, on the composition of the copolymer, copolymers having a high content of acrylonitrile units have been proposed for improving the performance characteristics of the carbon fiber. As the spinning process, dry jet wet spinning process and wet spinning process have been proposed.
The dry jet wet spinning process entails a larger production cost than does the wet spinning process. Therefore, considering the production cost, the wet spinning process is employed. However, since fiber obtained by the wet spinning has a low denseness of structure and a large amount of fuzz, carbon fiber obtained by pyrolysis said fiber are generally not sufficient in mechanical properties and quality. In addition, the wet spinning is disadvantageous in that filament breakage in the tow often occurs during spinning.
When carbon fiber is produced by the pyrolysis of precursor fiber, the precursor fiber is subjected to oxidation in an oven and then carbonized in an inert atmosphere. Therefore, when an acrylonitrile copolymer is chosen as a material for the precursor fiber, there should be sufficiently considered not only its spinnability into fiber but also its thermochemical reaction characteristics in the oxidation and carbonizing steps and the performance characteristics of the carbon fiber.
In detail, the most suitable range of a composition of copolymer for the precursor fiber should be determined after due consideration of the facilitation of cyclization reaction, the prevention of fusing between filaments and the reduction of treatment time in the oxidation step; and the yield of the carbon fiber based on the precursor fiber, and the tensile strength, tensile modulus of elasticity and elongation of the carbon fiber after the carbonizing treatment. However, there are very few reports which quantitatively describe the composition of a suitable copolymer as an industrially valuable, general information.
Examples of information given by conventional proposals are summarized below. As an acrylonitrile copolymer for a carbon fiber precursor; a polymer having an content of acrylonitrile units of a certain value or more (about 90% by weight or more) is preferable. For passage through an oxidation step in a short time, it is effective to introduce a suitable reaction-initiating group, i.e., a functional group which accelerates the cyclic condensation reaction of a nitrile group (e.g. a carboxyl group). Under these conditions, a final composition of polymer is attained, for example, by adding other comonomers for facilitating the shaping into precursor fiber. Thus, there are a few qualitative information alone.
For example, a polymer having a high content of acrylonitrile units has a low solubility in solvents. Therefore, extremely limited processes are unavoidably employed for producing precursor fiber from the polymer, and the concentration of a spinning solution is low. Accordingly, the spinnable and shapeable properties of the polymer and the performance characteristics of carbon fiber obtained from the precursor fiber have heretofore been not satisfactory enough.
In the case of a polymer obtained by increasing the content of comonomer units for extending freedom in spinning and shaping, fusing between filaments tends to occur in pyrolysis of precursor fiber obtained from the polymer, and moreover the yield from carbonization is decreased. Thus, this polymer is still insufficient in passability through the process of pyrolysis and the quality and performance characteristics of carbon fiber.
There are very few reports which suggest the composition of a starting polymer which is free from the above various problems and permits or is advantageous for the oxidation step in a shorter time.
For example, the following methods have been proposed: a method in which the oxidation rate and the yield from carbonization are improved by employing a composition of polymer which gives a high cyclization and oxidation reactivity in the early stage of pyrotysis (Japanese Patent Application Kokoku No. 47-33019); a method in which the carbonization time is reduced in consideration of the stability in polymer production and a spinning step, by limiting a composition of polymer, for example, by employment of a vinyl carboxylate monomer (Japanese Patent Application Kokoku No. 51-7209); and a method in which an amine or a peroxide is added to a starting polymer (Japanese Patent Application Kokoku No. 51-7209 and Japanese Patent Application Kokai No. 48-87120).
However, in all of these methods, the range of the composition of polymer, i.e., the kind and content of comonomers, is wide, and it cannot be said that suitable carbonization characteristics of precursor fiber, etc. are chosen. In addition, it is considered that the acceleration of reaction in flameresisting permits high-speed carbonization, but the acceleration tends to deteriorate the performance characteristics of carbon fiber obtained by the carbonization. Thus, there have not yet been obtained a composition of polymer which is satisfactory with respect to both productivity and performance characteristics of carbon fiber. The addition of an amine or a peroxide to a polymer has various undesirable influences on the stability of a spinning solution and precursor fiber and hence is not an industrially excellent method.
In such circumstances, Japanese Patent Application Nos. 48-87120 and 52-34027 have proposed precursor fibers having a composition of polymer of an acrylonitrile/acrylamide/methacrylic acid ternary copolymer. In detail, the former discloses precursor fiber obtained from a copolymer of acrylonitrile/acrylamide/methacrylic acid in a ratio of 96/3/1 (wt %), and the latter discloses precursor fiber obtained from a copolymer of acrylonitrile/acrylamide/methacrylic acid in a ratio of 95.5/3.0/1.5 (mole %), i.e., 93.7/3.9/2.4 (wt %).
However, in the compositions of polymer of the precursor fibers disclosed in these references, the total proportion of acrylamide units and methacrylic acid units is excessive. When these precursor fibers are subjected to oxidation, the oxidation of precursor fiber proceeds rapidly in their surface portion but slowly in the inside of the filament. The thermally stabilizied fiber thus obtained has a cross section with heterogeneous structure in the radial direction of a filament in which the inside is not sufficiently oxidized. This tendency becomes marked when the oxidation is tried to be carried out in a short time. It is difficult to obtain carbon fiber with a high tensile modulus from the thermally stabilized fiber having a cross section with such a heterogeneous structure.
On the other hand, Japanese Patent Application Kokai No. 63-35821 discloses precursor fiber having a high iodine adsorption capacity, but this precursor fiber is obtained substantially from two components, i.e., acrylonitrile and itaconic acid and hence is different in comonomers from the precursor fiber of the present invention. In addition, the content of acrylonitrile units in the precursor fiber disclosed in the above reference is higher than that in the precursor fiber of the present invention, namely, it is substantially 99% by weight or more.
In the process disclosed in the above Japanese Patent Application Kokai No. 52-34027, the oxidation time is as long as 50 to 100 minutes, but carbon fiber obtained in this case has a tensile strength of 300 kg/mm.sup.2 or less. Also in the process disclosed in the above Japanese Patent Application Kokai No. 48-87120, the oxidation time is as long as 40 minutes, but carbon fiber obtained in this case has a tensile strength of 400 kg/mm.sup.2 or less.
Thus, although precursor fibers of an acrylonitrile/acrylamide/methacrylic acid ternary copolymer have been proposed, there has not-yet been known precursor fiber from which high-performance carbon fiber can be produced for a short oxidation time in the process of pyrolysis.
There has not yet been known a technique of producing, by wet spinning, precursor fiber which does not show filament breakage for a long period of time and has only a small amount of fuzz, by using as spinning material such a ternary copolymer as is described above.